There has been developed use of a light source of a shorter wavelength, and a focusing lens having a larger numerical aperture (hereinafter, simply called as “NA”) in order to increase the recording capacity of an optical disk. The wavelength of the light source and NA of the focusing lens used in DVDs are generally 650 nm and 0.6, respectively. There has been proposed an optical system for optical disks of future generation in which the wavelength of the light source is 405 nm, and NA of the focusing lens is 0.85. Technology is being developed regarding multi-layered optical disks constructed such that a number of data layers are laminated one over another at a certain interval in the thickness of the optical disk in an attempt to further increase the recording capacity of the optical disk.
Increase of NA of a focusing lens may resultantly increase spherical aberration relative to a variation (unevenness) in thickness of a substrate of an optical disk. The thickness of the substrate herein means a thickness of the substrate from the light receiving plane of the optical disk to the recording layer thereof. Since spherical aberration due to substrate thickness variation is proportional to the fourth power of NA, a spherical aberration of 10 mλ (=0.01 λ) is generated as the substrate thickness is varied by 1 μm in the optical system in which the wavelength of the light source is 405 nm, and NA of the focusing lens is 0.85. A coma aberration, which is generated by tilting of the optical disk relative to the optical axis of the focusing lens, is increased, as NA is increased, even with the same tilting amount. A spherical aberration or a coma aberration may degrade data recording characteristics. Accordingly, it is a common practice to detect an aberration of a light spot focused by the focusing lens, and control the output of the light source so as to compensate for the recording characteristics. This conventional art is disclosed, for example, in Japanese Unexamined Patent Publication No. 2001-160233 (patent document 1).
There has been known, as a method for detecting spherical aberration, a technology of dividing received light reflected from an optical disk into several areas on a cross-sectional flat plane of the light beam, and detecting a focus error signal with respect to each of the areas to calculate aberration. This technology is disclosed, for example, in Japanese Unexamined Patent Publication No. 2000-182254 (patent document 2).
Regarding coma aberration, there has been known a technology of detecting tilting of an optical disk with use of a tilt sensor provided in an optical head device, and calculating aberration based on the detection result. Regarding a multi-layered optical disk provided with multiple data layers, there is proposed an arrangement in which an aberration compensator is provided to compensate for spherical aberration with respect to each of the data layers, in light of the fact that the substrate thicknesses differ from each other with respect to the data layers. Examples of the aberration compensator are: the one in which a transparent plate member is provided between the focusing lens and the optical disk for compensating spherical aberration; the one in which wedge-like transparent blocks are assembled together to set the optical path length from the focusing lens to the respective data layers identical; and the one in which a diverging lens and a converging lens are arranged at respective appropriate positions between the focusing lens and the collimator lens for making laser beams from the light source into parallel light beams, and the distance between the diverging and converging lenses is rendered variable by a voice coil motor for compensating spherical aberration. These aberration compensators are, for example, disclosed in Japanese Patent No. 2502884 (patent document 3).
The working distance corresponding to the distance between the focusing lens and the optical disk is from 0.2 to 0.6 mm when NA of the lens to be used is 0.85. Therefore, it is difficult to arrange a plate member or wedge-like blocks between the focusing lens and the optical lens, considering vertical displacement of the optical disk arising from rotation of the optical disk, or vibrations exerted from the outside. In view of this, there is generally provided an aberration compensator between the collimator lens and the focusing lens. In the arrangement, recording characteristics of the multi-layered optical disk are corrected by compensating the spherical aberration with respect to each of the data layers before controlling the output of the light source based on the detected aberration amounts.
Aberration correcting means such as an aberration compensator for compensating the aberration with respect to each of the data layers is required in the optical head device for use in recording/reproducing data on an optical disk having multiple data layers. The aberration correcting means is adapted to reduce the aberration, which is supposed to be generated in applying the focusing lens designed such that the aberration is set to 0 with respect to a specific substrate thickness, to a data layer having a substrate thickness different from the specific substrate thickness. The aberration correcting means is driven to minimize the aberration amount detected by the aberration detecting means provided in the optical head device. Let us assume an arrangement in which a third-order spherical aberration is to be detected, and the aberration correcting means is so designed as to reduce such a third-order spherical aberration. Such an arrangement makes it possible to set the third-order spherical aberration to 0 with respect to any data layer by controlling the aberration correcting means to make laser light incident on the focusing lens into converging light or diverging light. Despite such a merit, however, the above arrangement fails to set the total aberration including aberration of the fifth and higher orders to 0, with the total aberrations with respect to the data layers being different from each other. In this way, if the above proposed arrangement regarding aberration detection and aberration reduction is applied to the multi-layered optical disk having multiple data layers, a detected aberration amount and an actual aberration amount are different from each other. A similar drawback should be considered, as the order of aberration to be detected is raised from the fifth order to the seventh order or the like, as long as undetectable aberration of a higher order remains. Therefore, the recording characteristics compensating method in which the output of the light source is controlled based on the detected aberration amount, as having been employed in the conventional art, fails to carry out optimum recording characteristics compensation, because output control is not executed when the detected aberration amount of a low order is 0 although there actually remain aberrations of a higher order which are different from each other with respect to the data layers. Furthermore, according to the conventional method, information is required as to the layer number of the target data layer, in addition to information relating to the detected aberration amount, and it is required to optimize the recording power based on such information. Thus, the conventional arrangement not only necessitates a program for learning a relation between the aberration amount and the optimum recording power with respect to each of the data layers, and for storing the learning results, but also makes the program complicated.
More specifically, if the aberration detecting means for detecting the third-order spherical aberration is used, and aberration correction is implemented with use of the aberration compensator based on the third-order spherical aberration amount detected by the aberration detecting means in recording/reproducing data on the first data layer and the second data layer whose distances from the optical disk surface (light receiving plane) are different from each other, it is more likely that a relation between the detected third-order spherical aberration amount and the optimum recording compensation amount, namely, correction residual with respect to the data layers may be varied from each other.
FIG. 11 shows a relation between substrate thickness variation, and third-order spherical aberration, and total aberrations, based on the substrate thickness of the optical disk as a parameter. The total aberration herein means aberration including third-order spherical aberration and aberration of the order higher than the third order. In FIG. 11, the aberration is compensated by the aberration compensator such that the third-order spherical aberration is 0 when the substrate thickness variation is 0. The substrate thickness variation in FIG. 11 is a variation relative to the respective initial thicknesses of the first and second data layers (e.g., 100 μm for the first data layer, and 110 μm for the second data layer), i.e., a relative value in thickness. Furthermore, the variation is not an average of thickness variation, as represented by “rms” or a like unit, but is an instantaneous value. As is obvious from FIG. 11, aberration amounts of the order higher than the third order are different from each other between the first data layer and the second data layer, although the third-order spherical aberration amounts are identical to each other between the first and second data layers. Even if the aberration detecting means acquires aberration amounts of the higher orders such as the fifth order, seventh order or the like, there still remains a difference in aberration component of the order higher than a highest order detectable by the aberration detecting means between the first and second data layers. Consequently, since the relation between the aberration amount detected by the aberration detecting means, and the optimum recording compensation amount is different from each other with respect to the data layers, it is required to provide a program corresponding to learning means (not shown) that learns a relation between the aberration amount and the optimum recording compensation amount with respect to each of the data layers in advance for recording compensation, and stores the learning results therein. In this way, the technical field of the present invention has encountered problems such as increase of learning hours and increase of the quantity of the program, with increase in the number of data layers.